The present invention relates to a method for joining at least two plate-shaped workpieces of similar or different materials by means of a fastening tool, and a device for operating such a fastening tool. Preferably the fastening tool is a riveting tool for setting the self-piercing rivets or a clinching tool for performing a clinching operation.
Various types of drives for fastening tools such as self-piercing riveting and clinching tools have become known. The most common type of drives comprises a hydraulic piston-cylinder-assembly for actuating a punch to perform the joining operation and a further hydraulic piston-cylinder-assembly for actuating a clamp to exert a clamping force upon the workpieces during the joining operation, cf: for example WO93/24256 and EP 0675774. In the method and device of EP 0675774 a “substantial” clamping force is exerted prior to and during the joining operation; as said in this publication the clamping force may be up to 1.5 tons. In practise clamping forces in the order of e.g. 8 to 10 kN are used. Even though this method does have its advantages, exerting a high clamping force prior and during the joining operation does have also some drawbacks. For example the high clamping force exerted during the joining operation may prevent free deformation of the self-piercing rivet. Furthermore, exerting a high clamping force prior to the joining operation may also suffer from some drawbacks in a combined method of using rivets and adhesive to join the workpieces because the high clamping force may detrimentally affect compression and flow of the adhesive from the joining area.
In a prior self riveting apparatus of applicant a small clamping force was exerted by means of a spring during the joining operation, and the clamping force was increased towards the end of the joining operation by having the piston of the hydraulic cylinder of the punch transfer a part of the punch force to the clamp via abutment means. The above mentioned EP 0675774 B1 discloses a similar apparatus wherein before and during the joining operation a separate hydraulic cylinder exerts a “substantial” clamping force which is momentarily increased at the end of the riveting operation to about 5 tons by abutment means between the piston of the punch and the piston of the clamp.
By WO 00/29145 it has become known to withdraw the punch after the joining operation and, when the punch has been withdrawn, to exert a clamping force upon the workpieces so as to reduce deformations of the plate-shaped workpieces (sheets) out of their plane. To this end this document discloses two basic principles for achieving this result. According to one principle a retaining device for the clamp is provided to prevent yielding of the clamp when the lower leg of the C-shaped frame as used in rivet setting machines springs back as a result of the punch having been withdrawn. According to the other principle the punch and the clamp, during the joining operation, are pressurized simultaneously by a high pressure via abutment means, and after the joining operation the punch is withdrawn whereupon the clamp is again pressurized so as to exert a relatively high clamping force. Furthermore, this document mentions that during the joining operation a high or small clamping force or no clamping force at all may be exerted.
Finally, it has become known to exert, prior to the joining operation, at least a small clamping force so as to urge the workpieces against each other and to prevent them from sliding relative to each other and in particular to perform rivet and workpiece checking operations, cf. for example WO 93/24258.